implemented ParallelHelper.IterateRows()

src/ImageSharp/Common/Helpers/ParallelFor.cs

@@ -1,63 +1,12 @@
 using System;
 using System.Buffers;
-using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 
 using SixLabors.ImageSharp.Memory;
 using SixLabors.Memory;
-using SixLabors.Primitives;
 
 namespace SixLabors.ImageSharp
 {
-    internal readonly struct RowInterval
-    {
-        public RowInterval(int min, int max)
-        {
-            this.Min = min;
-            this.Max = max;
-        }
-
-        /// <summary>
-        /// Gets the INCLUSIVE minimum
-        /// </summary>
-        public int Min { get; }
-
-        /// <summary>
-        /// Gets the EXCLUSIVE maximum
-        /// </summary>
-        public int Max { get; }
-    }
-
-    internal static class ParallelHelper
-    {
-        public static void IterateRows(in Rectangle rectangle, Configuration configuration, Action<RowInterval> body)
-        {
-            int maxSteps = (int)Math.Ceiling(
-                (float)(rectangle.Width * rectangle.Height) / configuration.MinimumPixelsProcessedPerTask);
-
-            int numOfSteps = Math.Min(configuration.MaxDegreeOfParallelism, maxSteps);
-
-            int step = rectangle.Height / numOfSteps;
-
-            var parallelOptions = new ParallelOptions() { MaxDegreeOfParallelism = numOfSteps };
-
-            int bottom = rectangle.Bottom;
-
-            Parallel.For(
-                0,
-                numOfSteps,
-                parallelOptions,
-                i =>
-                    {
-                        int yMin = i * step;
-                        int yMax = Math.Min(yMin + step, bottom);
-
-                        var rowInterval = new RowInterval(yMin, yMax);
-                        body(rowInterval);
-                    });
-        }
-    }
-
     /// <summary>
     /// Utility methods for Parallel.For() execution. Use this instead of raw <see cref="Parallel"/> calls!
     /// </summary>

src/ImageSharp/Common/ParallelUtils/ParallelExecutionSettings.cs

@@ -0,0 +1,36 @@
+// Copyright(c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Threading.Tasks;
+
+namespace SixLabors.ImageSharp.ParallelUtils
+{
+    /// <summary>
+    /// Defines execution settings for methods in <see cref="ParallelHelper"/>.
+    /// </summary>
+    internal struct ParallelExecutionSettings
+    {
+        public ParallelExecutionSettings(int maxDegreeOfParallelism, int minimumPixelsProcessedPerTask)
+        {
+            this.MaxDegreeOfParallelism = maxDegreeOfParallelism;
+            this.MinimumPixelsProcessedPerTask = minimumPixelsProcessedPerTask;
+        }
+
+        public ParallelExecutionSettings(int maxDegreeOfParallelism)
+            : this(maxDegreeOfParallelism, 2048)
+        {
+        }
+
+        /// <summary>
+        /// Gets the value used for initializing <see cref="ParallelOptions.MaxDegreeOfParallelism"/> when using TPL.
+        /// </summary>
+        public int MaxDegreeOfParallelism { get; }
+
+        /// <summary>
+        /// Gets the minimum number of pixels being processed by a single task when parallelizing operations with TPL.
+        /// Launching tasks for pixel regions below this limit is not worth the overhead.
+        /// Initialized with 2048 by default, the optimum value is operation specific.
+        /// </summary>
+        public int MinimumPixelsProcessedPerTask { get; }
+    }
+}

src/ImageSharp/Common/ParallelUtils/ParallelHelper.cs

@@ -0,0 +1,95 @@
+// Copyright(c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+
+using SixLabors.ImageSharp.Memory;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.ParallelUtils
+{
+    /// <summary>
+    /// Utility methods wrapping Parallel.For() execution optimized for image processing.
+    /// Use this instead of direct <see cref="Parallel"/> calls!
+    /// </summary>
+    internal static class ParallelHelper
+    {
+        /// <summary>
+        /// Get the default <see cref="ParallelExecutionSettings"/> for a <see cref="Configuration"/>
+        /// </summary>
+        public static ParallelExecutionSettings GetParallelSettings(this Configuration configuration)
+        {
+            return new ParallelExecutionSettings(configuration.MaxDegreeOfParallelism);
+        }
+
+        /// <summary>
+        /// Gets a span for all the pixels in <paramref name="buffer"/> defined by <paramref name="rows"/>
+        /// </summary>
+        public static Span<T> GetMultiRowSpan<T>(this Buffer2D<T> buffer, in RowInterval rows)
+            where T : struct
+        {
+            return buffer.Span.Slice(rows.Min * buffer.Width, rows.Height * buffer.Width);
+        }
+
+        /// <summary>
+        /// Iterate through the rows of a rectangle in optimized batches defined by <see cref="RowInterval"/>-s.
+        /// </summary>
+        public static void IterateRows(Rectangle rectangle, Configuration configuration, Action<RowInterval> body)
+        {
+            ParallelExecutionSettings parallelSettings = configuration.GetParallelSettings();
+
+            IterateRows(rectangle, parallelSettings, body);
+        }
+
+        /// <summary>
+        /// Iterate through the rows of a rectangle in optimized batches defined by <see cref="RowInterval"/>-s.
+        /// </summary>
+        public static void IterateRows(Rectangle rectangle, in ParallelExecutionSettings parallelSettings, Action<RowInterval> body)
+        {
+            int maxSteps = DivideCeil(rectangle.Width * rectangle.Height, parallelSettings.MinimumPixelsProcessedPerTask);
+
+            int numOfSteps = Math.Min(parallelSettings.MaxDegreeOfParallelism, maxSteps);
+
+            // Avoid TPL overhead in this trivial case:
+            if (numOfSteps == 1)
+            {
+                var rows = new RowInterval(rectangle.Top, rectangle.Bottom);
+                body(rows);
+                return;
+            }
+
+            int verticalStep = DivideRound(rectangle.Height, numOfSteps);
+
+            var parallelOptions = new ParallelOptions() { MaxDegreeOfParallelism = numOfSteps };
+
+            Parallel.For(
+                0,
+                numOfSteps,
+                parallelOptions,
+                i =>
+                    {
+                        int yMin = rectangle.Top + (i * verticalStep);
+                        int yMax = Math.Min(yMin + verticalStep, rectangle.Bottom);
+
+                        var rowInterval = new RowInterval(yMin, yMax);
+                        body(rowInterval);
+                    });
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static int DivideCeil(int dividend, int divisor)
+        {
+            // TODO: Is there a more efficient way to calculate this?
+            int result = dividend / divisor;
+            return dividend % divisor == 0 ? result : result + 1;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static int DivideRound(int dividend, int divisor)
+        {
+            return (dividend + (divisor / 2)) / divisor;
+        }
+    }
+}

src/ImageSharp/Common/ParallelUtils/RowInterval.cs

@@ -0,0 +1,35 @@
+// Copyright(c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.Memory;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.ParallelUtils
+{
+    /// <summary>
+    /// Represents an interval of rows in a <see cref="Rectangle"/> and/or <see cref="Buffer2D{T}"/>
+    /// </summary>
+    internal readonly struct RowInterval
+    {
+        public RowInterval(int min, int max)
+        {
+            this.Min = min;
+            this.Max = max;
+        }
+
+        /// <summary>
+        /// Gets the INCLUSIVE minimum
+        /// </summary>
+        public int Min { get; }
+
+        /// <summary>
+        /// Gets the EXCLUSIVE maximum
+        /// </summary>
+        public int Max { get; }
+
+        /// <summary>
+        /// Gets the difference (<see cref="Max"/> - <see cref="Min"/>)
+        /// </summary>
+        public int Height => this.Max - this.Min;
+    }
+}

src/ImageSharp/Configuration.cs

@@ -29,8 +29,6 @@ namespace SixLabors.ImageSharp
 
         private int maxDegreeOfParallelism = Environment.ProcessorCount;
 
-        private int minimumPixelsPerTask = 2048;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="Configuration" /> class.
         /// </summary>
@@ -77,24 +75,6 @@ namespace SixLabors.ImageSharp
             }
         }
 
-        /// <summary>
-        /// Gets or sets the minimum number of pixels being processed by a single task when parallelizing operations with TPL.
-        /// It's not worth to launch tasks processing pixels below this limit. Initialized with 2048 by default.
-        /// </summary>
-        public int MinimumPixelsProcessedPerTask
-        {
-            get => this.minimumPixelsPerTask;
-            set
-            {
-                if (value <= 0)
-                {
-                    throw new ArgumentOutOfRangeException(nameof(this.MinimumPixelsProcessedPerTask));
-                }
-
-                this.minimumPixelsPerTask = value;
-            }
-        }
-
         /// <summary>
         /// Gets the currently registered <see cref="IImageFormat"/>s.
         /// </summary>

src/ImageSharp/ImageSharp.csproj.DotSettings

@@ -0,0 +1,3 @@
+<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=common/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/CodeInspection/NamespaceProvider/NamespaceFoldersToSkip/=common_005Cexceptions/@EntryIndexedValue">True</s:Boolean></wpf:ResourceDictionary>

tests/ImageSharp.Tests/Helpers/ParallelHelperTests.cs

@@ -3,9 +3,8 @@
 
 using System.Threading;
 
-using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.ParallelUtils;
 using SixLabors.Primitives;
-
 using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Helpers
@@ -15,29 +14,31 @@ namespace SixLabors.ImageSharp.Tests.Helpers
         [Theory]
         [InlineData(1, 0, 100, -1, 100)]
         [InlineData(2, 0, 9, 5, 4)]
+        [InlineData(4, 0, 19, 5, 4)]
         [InlineData(2, 10, 19, 5, 4)]
         [InlineData(4, 0, 200, 50, 50)]
         [InlineData(4, 123, 323, 50, 50)]
+        [InlineData(4, 0, 1201, 300, 301)]
         public void IterateRows_OverMinimumPixelsLimit(
             int maxDegreeOfParallelism,
             int minY,
             int maxY,
-            int expectedStep,
-            int expectedLastStep)
+            int expectedStepLength,
+            int expectedLastStepLength)
         {
-            Configuration cfg = Configuration.Default.ShallowCopy();
-            cfg.MinimumPixelsProcessedPerTask = 1;
-            cfg.MaxDegreeOfParallelism = maxDegreeOfParallelism;
+            var parallelSettings = new ParallelExecutionSettings(maxDegreeOfParallelism, 1);
 
-            var rectangle = new Rectangle(0, minY, 10, maxY);
+            var rectangle = new Rectangle(0, minY, 10, maxY - minY);
 
             int actualNumberOfSteps = 0;
-            ParallelHelper.IterateRows(rectangle, cfg,
+            ParallelHelper.IterateRows(rectangle, parallelSettings,
                 rows =>
                     {
                         Assert.True(rows.Min >= minY);
+                        Assert.True(rows.Max <= maxY);
+
                         int step = rows.Max - rows.Min;
-                        int expected = rows.Max < maxY ? expectedStep : expectedLastStep;
+                        int expected = rows.Max < maxY ? expectedStepLength : expectedLastStepLength;
 
                         Interlocked.Increment(ref actualNumberOfSteps);
                         Assert.Equal(expected, step);
@@ -45,33 +46,38 @@ namespace SixLabors.ImageSharp.Tests.Helpers
 
             Assert.Equal(maxDegreeOfParallelism, actualNumberOfSteps);
         }
+        
 
         [Theory]
         [InlineData(2, 200, 50, 2, 1, -1, 2)]
         [InlineData(2, 200, 200, 1, 1, -1, 1)]
-        [InlineData(4, 200, 100, 4, 2, 1, 1)]
+        [InlineData(4, 200, 100, 4, 2, 2, 2)]
         [InlineData(4, 300, 100, 8, 3, 3, 2)]
+        [InlineData(2, 5000, 1, 4500, 1, -1, 4500)]
+        [InlineData(2, 5000, 1, 5000, 1, -1, 5000)]
+        [InlineData(2, 5000, 1, 5001, 2, 2501, 2500)]
         public void IterateRows_WithEffectiveMinimumPixelsLimit(
             int maxDegreeOfParallelism,
             int minimumPixelsProcessedPerTask,
             int width,
             int height,
             int expectedNumberOfSteps,
-            int expectedStep,
-            int expectedLastStep)
+            int expectedStepLength,
+            int expectedLastStepLength)
         {
-            Configuration cfg = Configuration.Default.ShallowCopy();
-            cfg.MinimumPixelsProcessedPerTask = minimumPixelsProcessedPerTask;
-            cfg.MaxDegreeOfParallelism = maxDegreeOfParallelism;
+            var parallelSettings = new ParallelExecutionSettings(maxDegreeOfParallelism, minimumPixelsProcessedPerTask);
 
             var rectangle = new Rectangle(0, 0, width, height);
 
             int actualNumberOfSteps = 0;
-            ParallelHelper.IterateRows(rectangle, cfg,
+            ParallelHelper.IterateRows(rectangle, parallelSettings,
                 rows =>
                     {
+                        Assert.True(rows.Min >= 0);
+                        Assert.True(rows.Max <= height);
+
                         int step = rows.Max - rows.Min;
-                        int expected = rows.Max < height ? expectedStep : expectedLastStep;
+                        int expected = rows.Max < height ? expectedStepLength : expectedLastStepLength;
 
                         Interlocked.Increment(ref actualNumberOfSteps);
                         Assert.Equal(expected, step);